A fuel cell essentially consists of two catalyst electrodes and a solid electrolyte membrane sandwiched between the electrodes. Hydrogen, the fuel, is ionized at one of the electrodes, and the hydrogen ions diffuse through the solid electrolyte membrane and combine with oxygen at the other electrode. When the two electrodes are connected through an external circuit, an electric current flows and electric power is supplied to the external circuit. Here, the solid electrolyte membrane has functions to diffuse the hydrogen ions, as well as to physically isolate the fuel gas (hydrogen) and oxygen and to block the flow of electrons.
It is accepted that the solid electrolyte membranes diffuse hydrogen ions through water clusters in hydrophilic channels (ion conducting channels). Therefore, the ion conductivity drastically lowers at low humidities by drying of water and at low temperatures by freezing of water. The quantity of water adsorbed and bound to ion conductive groups in the membrane and the channel structure formed by the ion conductive groups are considered very important for the ion conductivity.
The present inventors studied in view of the above problems in the background art and have arrived at a solid polymer electrolyte membrane that comprises an ion conductive polymer segment (A) and an ion nonconductive polymer segment (B), wherein ion conductive groups adsorb water of which a sufficient quantity shows melting temperatures in the range of −30 to 0° C., whereby water is prevented from drying at low humidities and from freezing at low temperatures and consequently the membrane can achieve sufficient proton conductivity even at low humidities and low temperatures. It has also been found that when the solid polymer electrolyte membrane has a morphology in which the ion conductive polymer segment (A) forms a continuous phase, the membrane achieves a sufficient quantity of water showing melting temperatures of −30 to 0° C.